1. Field of the Invention
The present invention relates to an image forming method by using a photothermographic material and, more particularly, it relates to an image forming method exhibiting high sensitivity and having excellent image storability, due to the combination of a photothermographic material having a specific sensitivity in a wavelength region of 350 nm or more and 420 nm or less, and a fluorescence intensifying screen having a high light-emitting property within the region described above.
2. Description of the Related Art
In recent years, there has been a strong demand to adopt a dry process for photographic development in the medical and printing and plate-making fields. This trend has come about due to various factors, such as environmental preservation and the need to use space more efficiently. In the fields described above, digitalization has rapidly progressed. Systems are now in place in which image data is loaded into computers, stored and optionally processed. The data or information is then outputted via transmission to thee required locations and printed onto photosensitive materials with laser image setters or laser imagers. Due to these systems, in situ image development and preparation have become quite common.
It is necessary that the photosensitive materials used in these systems are able to record images by high-illuminance laser exposure and that they can form clear black images having high resolution and sharpness. For such digital image recording materials, various types of hard-copy systems that utilize pigments and dyes, such as ink jet printers and electrophotographic systems, have been marketed as conventional image-forming systems. Nonetheless, these are not satisfactory in view of the image quality (i.e., sharpness, granularity, gradation and color tone) necessary for determining the diagnostic ability used in cases such as images for medical use. Further, the recording speed (i.e., sensitivity) has not yet reached a level capable of substituting existing silver salt films for wet development in medical use.
Thermal image-forming systems utilizing organic silver salts are well-known. In these systems, an image-forming layer in which a reducible silver salt (for example, organic silver salt), a photosensitive silver halide and, when necessary, a color-tone-adjusting agent for controlling the tone of silver are dispersed in a binder matrix.
After image exposure, the photothermographic material is heated to a high temperature (for example, 80° C. or higher) and black silver images are formed by an oxidation/reduction reaction between a silver halide or reducible silver salt (functioning as an oxidizer) and a reducing agent. The oxidation/reduction reaction is promoted by the catalytic effect of latent images of the silver halide that are generated by exposure. As a result, black silver images are formed in the exposed areas. These photothermographic materials have been disclosed in many publications and the Fuji Medical Dry Imager FM-DPL has been sold as a medical image-forming system for actual use.
Image-forming systems utilizing organic silver salts essentially encounter two serious problems, since these have no fixing step and the silver halide remains on the film even after heat development.
One of the problems involves image storability after development processing, particularly, the deterioration of the printout when exposed to light. A method of utilizing silver iodide has been known as a countermeasure for improving the printout quality. Silver iodide has a characteristic of causing considerably less deterioration of the printout compared with silver bromide or silver bromoiodide, since it has an iodide content of 5 mol % or less, and thus has great potential to aid in overcoming the problem. However, silver iodide particles known to date have extremely low sensitivity, making them far from suitable for use in actual systems. Further, there is a problem in that the excellent printout quality achieved with silver iodide is lost when a countermeasure for increasing sensitivity by preventing re-combination of photoelectrons and holes is applied.
As means for increasing the sensitivity of the silver iodide photographic emulsion, it has been well documented in professional literature that sensitivity is enhanced by dipping the photothermographic material in an aqueous solution of a halogen receptor such as sodium nitrite, pyrogallol and hydroquinone or silver nitrate, or by sulfur sensitization at pAg of 7.5. However, the inventors found that the halogen receptors exhibited very little sensitizing effect when used with the photothermographic material of the present invention, making them extremely insufficient.
Another problem is that, due to light scattering caused by residual silver halide, the film tends to become turbid and semi-transparent or opaque, thus lowering the image quality. In order to overcome this problem, a method of granulating the photosensitive silver halide to fine particles (the range in actual use being 0.15 μm to 0.08 μm) and reducing the addition amount as much as possible, thereby decreasing the turbidity caused by the silver halide, has been adopted. However, this compromise further lowered sensitivity, and could not completely solve the problem of turbidity, leaving clouding that made the film hazy.
In wet developing systems, residual silver halide is removed by treating the material with a fixing solution containing a solvent for the silver halide after development. As the solvent for the silver halide, various organic and inorganic compounds capable of forming complexes with silver ions have been known. Incorporation of similar fixing means was also attempted in the past for dry heat development. For example, it was proposed to incorporate a compound capable of forming a complex with silver ions in a film layer and solubilize the silver halide by heat development (usually referred to as “fixing”). However, this method requires the use of silver bromide or silver bromochloride, and also requires subsequent heating for fixing at high temperatures of 155° C. to 160° C. Further, a method of providing a separate sheet that contains a compound capable of forming a complex with silver ions (i.e., a fixing sheet), has also been proposed. In this method, images are formed by heat development of a photothermographic material, after which the fixing sheet is laid over the photothermographic meterial, thereby dissolving and removing the residual silver halide. However, since this system uses two sheets, it complicates the processing steps, and makes it difficult to ensure operation stability. Further, the fixing sheet has to be discarded after processing, resulting in waste materials. In light of these problems, it has been difficult to put this method into actual use.
In addition to the method described above, a fixing method in heat development has been proposed that incorporates a fixing agent for a silver halide in microcapsules and involves releasing and actuating the fixing agent during heat development, however, it is typically difficult to effectively release the fixing agent. While a method of fixing using a fixing solution after heat development has also been proposed, this is not suitable for a completely dry process since it requires wet processing.
As described above, all of the methods for improving clouding of the film known so far have significant drawbacks and have caused difficulties when put to actual use.
Meanwhile, it has been proposed to apply the aforementioned photothermographic material to photosensitive materials used in photography. The photosensitive materials used in pohtography referred to herein are not adapted to record image information by scanning exposure with a laser or the like, but rather a material on which images are recorded by planar exposure. This is generally used in the field of photosensitive materials for wet development and it has been known, for example, as direct or indirect X-ray films and mammographic films for medical use. These are also used for various kinds of photomechanical process films for printing use, recording films for industrial use, or photographic films for ordinary cameras. For example, X-ray photothermographic materials coated on both sides with a blue fluorescence-intensifying sheet; photothermographic materials using plate-shaped particles of silver bromoiodide; or photosensitive materials for medical use in which plate particles with high silver chloride content having (100) main face are coated on both surfaces of a support have also been disclosed in patent literature. However, none of these obtain the high sensitivity necessary for application to actual photosensitive materials for photography. Further, photothermographic materials that are coated on both sides are also disclosed in other patent literature (see, for example, JP-A No. 2002-90941). Japanese Patent No. 3229344 discloses a photothermographic material for X-ray photography using a UV-ray emitting fluorescent screen that emits light at 200 nm to 350 nm.
Each of the known examples uses fine silver halide particles of 0.1 μm or less, and although these do not exhibit worsened haze, they do show low sensitivity, making them unsuitable for actual use in photography.
While photosensitive materials using plate-shaped particles of silver iodide for the silver halide particles have been known in the field of wet development (see, for example, JP-A Nos. 59-119344 and 59-119350), there have been no examples of actual application thereof to photothermographic materials. This is because they tend to show less sensitivity, have no effective sensitizing means, and moreover, they encounter further higher technical problems, such as increases in haze due to enlargement of the particle size in the heat development.
JP-A Nos. 2003-215754 and 2003-91053 disclose exposing photothermographic materials, which use silver iodide as the silver halide, to high-illuminance light having an emission peak of 350 nm to 450 nm or to a laser. However, such high-illuminance light is not available in the X-ray exposure for photographic use to which the present invention is concerned. In addition, the inventions described in the patent literature set forth above are not suitable for actual photographic use since the silver halide particles are small and show less sensitivity, despite the fact that haze does not worsen.